Model airplanes, especially the small model airplanes powered by front-mounted motor, mostly fly on a small simple field, and typically, there is no flat runway for the small model airplanes to take off and land, and the propeller during flying is quite liable to be impacted by plenty of obstacles. In the prior art, what is generally adopted is the manner that the propeller is directly fixedly connected with the motor power output shaft (including motor main shaft or power output shaft of driven gear of motor-powered reducing gear train), therefore, during flying, especially in the process of takeoff and landing of the model airplane, once the propeller is impacted by an obstacle, in milder case, the propeller is broken, and in more serious case, the motor main shaft is bent to be out of work, and even the model airplane will be damaged.
In order to protect the propeller, some model amateurs use two screws for fixing a cylinder on the motor output shaft to form a propeller cushion, the propeller is sleeved on the propeller cushion, and a rubber band is sleeved on the two screws to tightly press the propeller on the propeller cushion, thus the motor drives the propeller to rotate during normal operation, and when the propeller is accidentally impacted, the rubber band tying the propeller is stretched out and broken under the action of impact moment and the propeller can be separated from the propeller cushion so as to protect the propeller to a certain degree, FIG. 1 is a schematic diagram of the connection of such a structure.
It can be seen that the propeller protecting device in the form of the rubber band is characterized by tightly pressing the propeller on the propeller cushion by using the rubber band, and the rotation of the motor drives the propeller by means of frictional force. Obviously, the transfer of rotational torque between the motor and the propeller requires quite large frictional force, and such a frictional force requires quite large pressure from the rubber band, however, when the tension force of the rubber band is relatively large, the propeller is hardly separated from the propeller cushion; in order to guarantee that the propeller is relatively easily separated from the propeller cushion when being impacted, the tension force of the rubber band tying the propeller cannot be too large. Therefore, two technical requirements in this method, i.e. transferring the rotational torque of motor and protecting the propeller from being impacted to further avoid damage, are contradictory in this simple technical proposal, so this technical measure is unsatisfactory in propeller protecting effect in practice.
Owing to the above problem, the development of front-mounted motor power model is substantially restricted, especially in the aspect of small-sized airplane.